This invention relates to the alignment of liquid crystal (LC) molecules in liquid crystal devices. Although the description which follows deals with liquid crystal displays (LCDs), the primary intended application, the invention is also useful in other devices such as optical shutters.
In a conventional LC cell the interior major surfaces of the glass plates, with electrodes deposited thereon, are each coated with a thin polymer. The polymer layers are rubbed (e.g., with a cloth) to orient the polymer chains or to form microscopic grooves along a predetermined direction. The LC molecules contacting the polymer align themselves along that direction by interacting with the polymer chains or the microscopic grooves. Thus, the polymer layers are known in the LC art as alignment coatings. In the absence of such layers, the LC molecules at the cell surface would tend to orient themselves randomly in contrast with having a preferred orientation established by the boundary conditions of the alignment coating.
Nematic LC materials, for example, can be easily aligned by coating both of the interior surfaces of the cell with a polyimide (usually a polymide) and by unidirectional rubbing of both of the polymer layers. If an appropriate crystalline polymer is used, such a surface treatment can also align smectic LC materials.
The orientation of LC molecules at surfaces is usually described in terms of the tilt bias angle (.alpha.) defined as the angle made between the liquid crystal director and the plane of the surface (see FIG. 2, inset). When .alpha. is zero or nearly zero degrees (i.e., .alpha.=0.degree..+-.5.degree.), the orientation is referred to as "homogeneous", and, when it is ninety or nearly ninety degrees (i.e., .alpha.=90.degree..+-.520 ), it is referred to as "homeotropic". It is generally a simple matter to produce LC orientations which are either homogeneous or homeotropic, but it has been much more difficult to achieve orientations with .alpha. between these extremes. For example, unidirectional homogeneous alignment is typically produced by rubbing a polymer-coated surface, and homeotropic alignment can be produced by treatment of a surface with a surfactant such as a silane or a chromium complex (J. Cognard, Molecular Crystal Liquid Crystal, Suppl. 1, p. 1 (1982)). There are several types of liquid crystal devices, however, which require high-tilt angles in the range of about 20-50 degrees: the super-twisted birefringent (SBE) LCD (T.J. Scheffer, et al., Society for Information display 85 Digest, p. 120 (1985)), the optical shutter pi-cell, (P.J. Bos, et al., Society for Information Display 83 Digest, p. 30, (1983)), the cholesteric bistable LCD (D.W. Berreman, et al., Applied Physics Letters, Vol. 37, p. 1072, (1980)), and the tilted-hybrid phase-change guest-host LCD (R.W. Filas, Society for Information Display 84 Digest, p. 206, (1984)). The conventional method for producing such high-tilt surfaces has been to employ high-vacuum evaporation of materials such as silicon monoxide at oblique evaporation angles. (E. Guyon, et al., Letters in Applied and Engineering Sciences, Vol. 1, p. 19, (1973)). SiO can also be used to produce homogeneous alignment. When the evaporation angle is 30.degree..+-.10.degree. to the plane of the surface, homogeneous alignment of LCs occurs in a direction orthogonal to the evaporation direction. Using this technique, H.L. Ong, et al., Journal of Applied Physics, Vol. 57, No. 2, p. 186, (1985), describe titled alignment of nematic LCs by using microscopically inhomogeneous surfaces. The surfaces consisted of small patches or islands of one material (i.e., the silane DMOAP) favoring homeotropic alignment surrounded by a matrix of another material (i.e., SiO) favoring homogeneous (planar) alignment. However, this procedure is time-consuming and can limit the substrate size which in turn limits the size of the display.
In view of the difficulties in achieving high-tilt angles, a chemical process would be highly desirable. The only previously known example of high-tilt produced chemically is given in a paper by G. Porte, Journal of Physics (Paris), Vol. 37, p. 1245 (1976). He observed that by treating a clean glass surface with a monolayer of aliphatic monoamines of varying alkyl chain length, n, the orientation of the nematic N-(p-methoxybenzilidene)-p'-butylaniline (MBBA) to be either tilted (6&lt;n&lt;10) or homeotropic (12&lt;n&lt;16). In his experiments, the liquid crystal was allowed to flow by capillarity into a cell, and the tilt direction was determined by the flow direction of the liquid. This method naturally produces antiparallel tilt (FIG. 5, p. 1247) on the two surfaces, which is not generally desirable. Although he claims that it is possible to produce uniform parallel tilt (FIG. 6, p. 1247) by heating to the isotropic phase and then cooling very slowly with a temperature gradient between the two surfaces, such an approach would not be practical for device applications.